1. Field of the Invention
This invention relates to the field of X-ray lithography. More particularly, this invention relates to a method for determining the development endpoint of an X-ray exposed resist by optically inspecting a test area while the resist develops.
2. Description of the Prior Art
Lithography is a process which creates patterns in a coating material after it has been applied on a surface. The coating material applied to the substrate is sometimes called a resist. During the process, selected portions of the resist are exposed to radiation. The radiation changes some property of the exposed portions of the resist. This change is later exploited in a developing step to create patterns of resist on the substrate by selectively removing portions of the resist from the substrate.
As an example, resists are typically sensitive to types of radiation energy, such as light, E beam, or X-rays. Exposure to this radiation can make selected areas of the resist more or less soluable in a certain type of solution called a developer. Patterns of resist are created on a substrate when more soluable parts of the exposed resist are dissolved from the substrate by the developing solution. This is the kind of process used to develop photographic film after it has been exposed to light in a camera.
Some lithographic processes, for instance those used in the manufacturing of electronic circuitry, pass radiation through a mask to selectively expose a resist. Portions of the mask permit radiation to pass through to expose some portions of the resist. Other portions of the mask prevent all or substantially all radiation from impinging on other portions of the resist. Consequently, due to the mask, some selected portions of the resist are exposed to more radiation (receive a higher dose) than other portions of the resist.
A developing step exploits the difference in properties between the irradiated and the unirradiated portions of the resist. Resist patterns, comprised of lines, are created when the developer (or developing step) removes selected parts of the resist from the substrate. For this description, lines of resist are defined as individual portions of resist which remain on the substrate after the lithographic process is complete.
During the developing step, a developer will remove, or etch, certain portions of the resist from the substrate faster than other portions. This selective removal during development results from the selective irradiation of the resist, i.e., different resist portions receiving radiation doses. The selective removal creates the resist pattern on the substrate.
Endpoint detection is a method for determining when to stop the developing process. The time chosen to stop the developing is called the development endpoint or endpoint. Endpoint detection is a critical determination in many lithographic processes. If the developing is stopped too early, the resist will not be removed, or cleared, from the substrate and therefore no pattern will be formed. In the other extreme, if the developing process is stopped too late, all the resist could be removed, or cleared, from the substrate, completely exposing a bare substrate so that no pattern remains.
Endpoint detection also effects the creation of the lines which form the resist pattern and appear like "hills or cliffs" on the substrate with cleared areas between the lines. Lines are formed during the developing step as the most soluable or easily developed portions of the resist clear from the substrate and the less soluable or harder-to-develop resist remains on the substrate. During development, the point in time when the first region on the substrate is exposed, i.e., is cleared, is called the first clear point. At this first clear point, the lines have a certain width. As the developing continues in time past the first clear point and the lines themselves are gradually removed, the linewidth changes, i.e. the lines become narrower. Poor endpoint detection can cause problems in creating the proper width of pattern lines, i.e., linewidth.
Endpoint detection is particularly critical if producing resist linewidths of a precise dimension is important as in the production of submicron linewidths on electronic circuitry. Choosing an endpoint too soon creates linewidths too wide and choosing an endpoint too late creates linewidths too narrow.
The prior art has many ways to detect endpoint development or resist clearing in lithography. These methods include: monitoring light reflected or diffracted from a test region; using optical interferometric techniques; looking for changes related to material properties (fluorescence or luminescence); detecting substrate reaction products in an etching plasma; and monitoring separate test samples undergoing the same development process as the original workpiece.
Some prior art uses reflection of light from a test region to detect development endpoint. U.S. Pat. No. 4,496,425 discloses an etched fresnel zone on an aluminum layer. Intensity of light reflected from the fresnel zone during plasma etching is monitored and the etching process terminates when the intensity falls below a predetermined level.
Some prior art uses diffraction of light from a test pattern to determine endpoint. (Diffraction is a change which light undergoes when it passes by the edges of opaque bodies or through narrow slits.) U.S. Pat. No. 4,142,107 discloses a visual inspection and automatic endpoint detection of residual photoresist. By exposing a test area to two different radiation levels, different patterns of exposure are created on the test area. During development, the more exposed areas develop faster forming the remaining resist into a changing, light diffracting, optical grating pattern on the test area. Endpoint is reached when the resist clears the test area.
Automatic endpoint detection is possible by monitoring light diffracting off the changing optical grating pattern. This method can be used with ultraviolet, X-ray and E beam exposure systems. U.S. Pat. Nos. 4,303,341 and 4,039,370 disclose exposing a diffraction grating pattern to light and monitoring intensity or ratios of intensities to determine undercutting of a layer or pattern dimensions. Similarly, U.S. Pat. No. 4,179,622 teaches exposing an optical grid pattern on a photoresist layer, directing a light ray on to the grid pattern, and looking for an intensity minimum of the 2nd diffraction order to indicate the end of the development process.
Optical interferometric techniques are also used to detect endpoints. For example, U.S. Pat. No. 4,998,021 discloses projecting of coherent light onto an upper surface layer and receiving superposed light caused by the interference of light reflected from two surfaces of the layer. Endpoint is determined by monitoring variations in the sampled light interference data.
As U.S. Pat. No. 4,717,446 states, there are many variations on this method including looking for sharp changes in signal slope, oscillation frequency, or level detected. However, many of these methods are difficult to use and prone to error. In particular, these methods produce error in X-ray lithography because X-ray radiation on a local scale is not uniform. This nonuniformity causes surface roughness during developing which in turn causes light to be reflected unpredictably.
Prior art exists which uses properties like fluorescence or luminescence to detect endpoint. U.S. Pat. No. 4,482,424, discloses doping (physically mixing) the resist with a fluorescent material. The monitored fluorescence intensity decreases as the resist is removed from the substrate. U.S. Pat. No. 4,377,436, discloses monitoring the luminescence associated with a reaction product of plasma etching in the immediate vicinity of etching.
Mixing additives in the resist material, as these methods do, often changes critical resist properties. Changes in the resist can result in many later process problems including: poor line width definition, nonvertical resist walls, and resist insolubility.
Some prior art discloses endpoint detection of plasma etching by monitoring changes in energy emissions from the plasma. (Plasma etching is a type of development process.) U.S. Pat. No. 4,482,424 describes a mass spectrometric analysis of the etching plasma that detects substrate reaction products which indicate that the substrate has been reached. Detection time delays cause inaccuracies in these methods. They also are not used in wet chemical developing.
Other prior art discusses monitoring a separate test sample to determine endpoint in an original workpiece. In U.S. Pat. No. 4,512,847, a test sample has the same material or structure as a workpiece but the sample has a wedge with an angle pointing toward the surface being removed. As the sample and workpiece are simultaneously exposed to the same subtractive process, the exposed gap between the walls of the wedge widens in the sample. A relationship exists between the gap size and material removed. U.S. Pat. No. 4,717,446 relates to the etch endpoint detection of epitaxially grown silicon using a separate monitor wafer, i.e., a second test sample.
These methods, which require a second test sample to determine endpoint, inspect the second sample to determine endpoint rather than the workpiece on which the desired resist pattern is produced. These methods are not as accurate as using in situ test samples (those on the original workpiece) because in situ test samples are more likely than remote test samples to experience the same process conditions as the original workpiece.
2. Statement of Problems With the Prior Art
Most of the prior art methods for detecting endpoint are relatively inaccurate, complicated, and subject to human interpretation. Some prior art results vary with changing process variables. Some prior art includes additives in the resist and/or uses a second comparable workpiece to determine endpoint. Most prior art does not teach how to predict multiple endpoints nor does it teach how to produce linewidths within tighter tolerances.
Optical interferometric techniques are difficult to use and prone to error. These techniques also require complicated monitoring equipment. Endpoint is often determined by operator judgement and its detection therefore is very dependent on operator skill, perception, and fatigue.
Prior art methods that etch diffraction pattern test fields on the resist, perform adequately to a point, however, these methods require sophisticated equipment to monitor light diffraction. Many of these methods are also subject to operator judgment.
Some disclosures required more than one irradiation of the wafer. Using only one irradiation of the workpiece saves time and is more efficient. Also, using one irradiation eliminates the possibility of unwanted dose level variations in subsequent irradiations.
Methods using fluorescence, luminescense, or measurement of some property change in a plasma require either additives to the resist or some etching of the substrate layer.
Most known methods involving reflection and diffraction patterns and most endpoint detection techniques using a single workpiece only predict an endpoint when the resist first clears from a substrate. These methods do not indicate how far the linewidth developing has progressed at each point in the developing process. Therefore, these methods predict linewidth only at one point in the development, usually the linewidth corresponding to the first clear of the resist.
Because most of this prior art does not teach how to accurately predict more than one development endpoint, there is no teaching about how to produce the resist linewidths that correspond to the many different endpoints which occur before or after reaching the only development endpoint predicted. An example illustrates how endpoint detection, limited to one endpoint, affects linewidth production. If the endpoint detection method predicts a development endpoint before the desired linewidth is reached, either the developing agent will not have yet cleared the resist from the substrate or the agent will not have laterally developed the resist long enough to produce the required linewidth. (Lateral thinning from developing occurs when the developing process removes resist from the side walls of the resist line.) On the other hand, if the endpoint is predicted after the desired linewidth is achieved, the linewidth will be degraded by too much lateral etching. (Wet bath etching degrades line width by excessive lateral etching of the resist walls. Ion etching methods degrade linewidth by excessive lateral etching and sputtering substrate deposits on the walls of the patterned resist.)
Furthermore, the prior art, known by the applicants, does not disclose any method to tighten line width tolerances. Tolerances designate the lowest and highest linewidth dimensions that are acceptable in the pattern formed on the substrate. To decrease the tolerances around a target linewidth, an endpoint detection method needs to be able to predict, as the resist develops, finer gradations of linewidth above and below the target linewidth. Most art teaches how to determine only one endpoint and therefore does not disclose how to determine gradations of linewidth during the developing step.
As desired linewidths become narrower, choosing the development endpoint for the desired linewidth tolerances becomes more difficult. This loss of linewidth control becomes particularly significant if pattern feature sizes are a few micrometers or smaller. A given amount of linewidth deviation will cause a greater percentage error when associated with a narrow line than the same given amount of deviation will cause when associated with a wider line.
To achieve increased circuit densities, newer integrated circuits and masks need patterns that have very narrower linewidths and lines within tighter dimensional tolerances. Producing these narrow widths precisely, requires more accurate and cost effective endpoint detection methods. This is particularly true with linewidths produced using higher energy radiation like X-rays. These linewidths are narrower than 1 micron (less than one millionth of a meter) and are currently undetectable by simple, cost effective methods.
Most of the prior art can only detect endpoints of linewidths on the order of 3 microns. Using the prior art to produce linewidths much smaller than this becomes very time consuming. Often the workpiece has to be removed from the developing process and observed under a Scanning Electron Microscope (SEM).